| Popis: |
This paper evaluates the impact of reservoir pressure on displacement efficiency when injecting CO2 injection in a super giant Abu Dhabi carbonate oil reservoir. The term Efficient Displacement Pressure (EDP) was defined to distinguish it from Minimum Miscibility Pressure (MMP). EDP has similar characteristics as MMP, resulting in very high recovery when the operating pressure exceeds EDP/MMP. The need for the EDP concept arose when evaluating feasibility of a CO2 injection project. During this evaluation, displacement by CO2 was found to be very efficient at pressures much lower than the saturation pressure, and multi-contact miscibility between the oil and the injectant could not be proven by classical slim tube experiments in the EDP pressure range. EDP was assessed using different analytical and modelling approaches in the absence of reliable experimental results: (i) Analytical means, (ii) Equation of State (EOS) PVT Softwares, (iii) 1D Compositional Numerical Modelling. Simulations were performed using a 10-component EOS, well tuned using a variety of advanced PVT experiments. The 1D model was designed to mimic a representative part of the field where the feasibility of the CO2 pilot project was being studied. Results from the different methods were analyzed and compared to properly assess the EDP range. Investigations showed that determining exact EDP values remains difficult, because the results strongly depend on the methodology used, the ability of EOS to accurately predict slim tube experiments, and the dependency of recovery factor on the grid size in numerical simulations. The sensitivity to water saturation and water blocking were not investigated here. However, in the defined EDP range, it was demonstrated that very high displacement efficiency and recovery can be achieved by injecting CO2 at an operating pressure in the identified EDP range. The recovery was found to be extremely high, even if the EDP range is below the saturation pressure, where miscibility is not necessarily guaranteed. This calls into question the general pre-requisite that operating above MMP is a necessary condition for the success of a CO2 injection project. |
